1. Field of the Invention
This invention relates in general, to electrical inductive apparatus and, more specifically, to three-phase transformers utilizing no-load tap changers.
2. Description of the Prior Art
No-load tap changers are used to provide a range of output voltages for transformers. Such transformers contain a series of connections or taps from various sections of either the primary or secondary winding such that a different number of turns of the winding are provided at each tap. Conventional no-load tap changers, which operate when the transformer is de-energized, include a plurality of stationary contacts, each connected to a different tap on the winding, and a contact assembly which can be moved to engage any one of the stationary contacts; thereby changing the turns ratio between the primary and secondary windings and altering the output voltage of the transformer. No-load tap changers for three-phase transformers have separate sets of stationary and movable contacts for each phase of the transformer. The movable contacts are usually connected together such that they may be rotated simultaneously.
As most no-load tap changers are mounted within the main transformer enclosure, there is a continuing need to minimize the size of such tap changers and thereby reduce the overall size and weight of the transformer itself. One of the main areas of concern, and one that consumes the most space in a tap changer, is the drive means used to switch the movable contacts between successive stationary contacts.
One type of drive means, known in the prior art, positions the axis of rotation of the operating force, operative from outside the transformer enclosure, parallel to the axes of rotation of the movable contacts in each phase of the tap changer. Tap changers utilizing this type of drive means are extremely large since the aligned connections between the operating force and the movable contacts require the circular arrangement of stationary contacts to be mounted perpendicular to the side of the core and coil assembly of the transformer.
As a means of reducing the space consumed by the tap changer within the transformer enclosure, it is known to position the tap changer above the case and coil assembly such that the circular arrangement of stationary contacts is parallel to the horizontal axis of the core and coil assembly. In such an arrangement, the movable contacts and the operating handle usually have different planes of rotation. A drive means, typically comprised of level or spur gears, and a motion transmitting means are used to translate motion between the different planes of rotation of the tap changer.
For example, in U.S. Pat. No. 3,396,248, issued to G. A. Wilson, Jr., the rotary operating force is translated to a different plane by a drive means comprised of level gears, which is connected to a motion transmitting means, consisting of level gears, at each phase of the tap changer, which further translate the rotary motion to the third non-aligned plane of rotation of the movable contacts.
Similarly, in a co-pending application, Ser. No. 599,241, now U.S. Pat. No. 4,013,847, in the name of E. Guidosh and assigned to the present assignee, the rotary operating force applied from outside the tank is translated by a rack and pinion drive means to linear movement of an operating mechanism, mounted in a different plane; which linear movement is further translated to rotary motion of the movable contacts by individual rack and pinion gears at each phase of the tap changer.
Similarly, in another co-pending application, Ser. No. 732,635, filed Oct. 15, 1976, now U.S. Pat. No. 4,035,717, in the name of E. Guidosh and assigned to the present assignee, a drive means comprised of a worm and worm gear translates motion to circular members at each phase of the tap changer, which are linked by a connecting member and which replace the individual rack and pinions of the previously cited application.
Regardless of the particular gear used in each type of tap changer, the same result is obtained; namely, the rotary operating force applied in one plane is translated to rotary motion of the movable contacts, mounted in a different plane, by a single drive means and individual motion transmitting means associated with each phase of the tap changer.
Although satisfactory in operation, tap changers with this type of linkage utilize large and rigid drive and connecting members to insure that the gear sets are properly aligned and also to withstand the forces involved in switching all three phases of the tap changer. In addition, backlash, inherent in all gear sets, necessitates that the stationary contact structures be enlarged to compensate for the buildup of tolerances along the gear drive train and to insure adequate engagement of the stationary and movable contacts.
Thus, further improvements and size reductions in no-load tap changers are still desirable and it is an object of this invention to provide a no-load tap changer for three-phase transformers which is smaller and more economical than tap changers known in the prior art and also to provide a no-load tap changer which will enable significant economics to be made in the design of the transformer itself.